One of the problems associated with battery powered devices is correctly estimating the remaining battery life. It is useful to be able to correctly estimate this parameter because it facilitates programming the replacement of the batteries. Such programs are particularly useful when the battery powered devices are part of a more complex system in which more than one device is to be monitored. For example, wireless fire detection systems can include hundreds of installed devices, and it is important to correctly program the maintenance activities.
Various problems are present in determining the remaining battery life of a system. Common problems can include one or more of the following issues. Battery performance can strongly depend on the environmental condition, especially temperature (the battery life is strongly reduced at low temperatures). The performance of a battery strongly depends on the type of usage, for example, high current sink for short periods or constant sink for long periods. Battery life depends on the aging of the battery. A battery can stay on the shelf an undetermined period before being installed. Additional problems include output voltage characteristics. The voltage output of some types of batteries, such as lithium ion types, is constant for most of their life, having a rapid fall of voltage with a sharp knee at the end of their life.
These above noted characteristics make it difficult to estimate the battery life by only measuring the output voltage or simply measuring the time from the installation. It is obvious that being too conservative in battery life estimation has a negative impact on the frequency of maintenance services as well as a negative impact on the environment. It is preferable that batteries that are not completely exhausted are not replaced too soon.